Some combat tricks I saw from VITA version [Non story spoilers]

[ArchmageXin]

Rean - He have the ability to delay enemies, even bosses. Combine with the fact his Master Quartz can increase his S-Craft bar fairly quickly, Rean can literally stunlock even bosses. (This is nerfed in CS2)

Fei -She quartz come with a 50% dodge. There is a buff called "Insight" which grant you 50% dodge. Combine the two and you get a Ninja tank that is completely immune to all melee attacks, including the ones from bosses as long as you have the Insight buff. Furthermore, when her HP gets low, she can get 5-6 attacks in a row.

Emma -While not cheesy per say, late game her damage is obscene. She should be maxxed level out and decked with best caster gear. She also have the best defensive ability (IIRC, Lunar blessing?) which allow her to cast a zero casting time spell reflection AND boost ally spell damage. In some boss fights where you absolutely will be 1 shotted if the boss got off a single spell, Emma is a must. She is also best girl :D

Laura. There is a Quartz called Domination which double her FIRST attack. Combine that with every + attack item and a S-craft and she can one shot anything. Still not good as Emma though :)

Meicha-His shot gun does AOE damage. While he will never out damage Laura, the AOE effect allow him to inflict a ton of conditions. Stack him end game and he is useful vs most non-boss characters.

As for the others- I never found anything special with them.

[MabinoZerk]

You can have Jusis spam Noble Command to boost your stats, but other than that I don't know how you can use the other characters to break the game.

[ArchmageXin]

The only use I found for the other people is to change their Quartz ASAP so they can actually be useful. Jusis and Gaius's are especially terrible. Meitcha's also need to be changed ASAP as well, I mean, a guy with a shotgun but a defensive Quartz...

[Stabbey]

The names in the English version are Fie and Machias, by the way.

[Monterossa]

Give all the best quarts to Fie and with her speed she'll always get the first turn and one-shot everything that isn't a boss.

[Sergei Lou]

Moebius Master Quartz > Chrono Burst > Stat Boosting Food > Break Everything.